Differential Protection

Yes the specific vector group input is necessary for the proper protection of the transformer...

https://www.microener.com/contenu/uploads/2012/08/DIF87_PRELIM-ME.pdf

There will be a phase shift between the HT side current and the LV side current, particularly in Delta/Star Transformers. If this is not taken care, even during normal loading conditions, the relay could see a differential current and if the setting is lower than this current, the relay would operate. And, yes! by programming a wrong vector group, the relay might operate for thro-fault conditions too.

Looks like you got the answer earlier. Anyway i may add that it actually depends. It depends on how far is the value of the set vector group from the real one.

Also the differential current's value varies with the load. Theoretically it should not but reallity beats theory. Since the set vector group differs from the real one, the relay calculates (on the assumption of a digital relay used) a different differential current than it should be. Once the treshold is reached the relay trips. Good luck with that.

Worse is not to trip when it should. Get fireworks as a result.

If you have already experienced a real trip with a wrong vector group I'd love to check the oscilo if you may share it with us.

thank you very much for all

I want to share my experience in this topic. i found that if we put vector group in wrong the relay will give nuisance trip.when i worked in 7.5 MW Power plant having 10MVA Generating Transformer. During commissioning the plant, The transformer trip by differential relay near to full load condition. Then we checked the relay and found setting problem. we put correct vector group and reached full load.

Why anyone would want to do this might forever remain a mystery, then.